1. Field of the Invention
The present invention relates to an injection molding machine controller, particularly to improvement of a controller for stabilizing measurement.
2. Description of the Prior Art
A conventional injection molding machine (1) compresses resin in a mold only for a set time at a predetermined dwell pressure after injection is completed (dwell step), (2) cancels the dwell pressure after the set time passes to start a measurement step in accordance with the rotation of a screw and continues the measurement step until the resin collects at the front end of an injection cylinder, and (3) retreats the screw up to a previously-set measurement completion position to complete the measurement step.
However, a screw position at the time of dwell completion is not always constant and thereby, the screw position may fluctuate. Moreover, because a dwell pressure is extremely high as compared to a set back pressure under measurement, a problem occurs that a screw rebounds and thereby retreats immediately after canceling the dwell pressure and changing the dwell step to the measurement step.
Moreover, the retreating distance due to rebound of the screw is not always constant because the distance changes depending on various factors such as degree of resin injection into a mold at the last shot and a gate sealing progress state.
Therefore, a problem occurs that a substantial measurement start position variously changes every shot, a measurement stroke, that is, the screw moving distance from the measurement start position up to a measurement completion position fluctuates, and thereby fluctuation occurs in the measurement state every shot.
The fluctuation influences the volume of a resin to be injected into a mold at the next shot and finally, causes a molding trouble such as fluctuation of the weight or specific gravity of a molded product.
Moreover, in case of some injection molding machines provided with a shut-off nozzle the measurement step is started by positively retreating a screw by a predetermined distance from a dwell completion position in order to lower the internal pressure of an injection cylinder and securely close a nozzle.
Also in this case, however, a measurement start position is set to a position retreating a predetermined distance from a previous dwell completion position. Therefore, fluctuation occurs at a final screw retreat position, that is, at a substantial measurement start position due to the fluctuation of the dwell completion position. Therefore, also in this case, a problem occurs that a measurement stroke fluctuates every shot similarly to the previous case.
Japanese Patent Application Laid-Open No. 4-241918 discloses a method for controlling an injection molding machine in which rebound of a screw caused at the time of switching from dwell step to measurement step is prevented and a measured amount of resin becomes stable.
In case of the above control method, however, the measurement stroke every shot does not resultantly become constant because the retreat of a screw is actually started from a dwell completion position and the retreat of the screw is stopped when the resin reaction acting on the screw becomes a set value or less to start measurement.
As described above, the causes are as follows: that is, because a dwell completion position itself fluctuates and moreover, the resin reaction acting on a screw after dwell is completed is changed every shot due to various factors such as degree of resin injection into a mold at the last shot and a gate sealing progress state, fluctuation occurs every shot also in the screw position when the resin reaction acting on the screw becomes a set value or less, that is, the screw position at the time of measurement start.
It is an object of the present invention to provide an injection molding machine controller capable of executing stable measurement by keeping a measurement stroke constant without being influenced by various factors such as degree of resin injection into a mold at the last shot and a gate sealing progress state.
To attain the above object, the present invention comprises measurement start position setting means for setting a measurement start position, measurement start position storing means for storing a measurement start position set by the measurement start position setting means, and measurement start position control means for starting the measurement step by moving a screw from a dwell completion position up to the measurement start position after the dwell step is completed.
Because the processing in the measurement step is executed in the screw moving range from a set measurement start position up to a measurement completion position, that is, a predetermined measurement stroke range, it is possible to execute stable measurement even if fluctuation occurs at the dwell completion position of the screw.